The closest thing that Mike Stamford could compare to the insidious joy of introducing someone to Sherlock Holmes, was standing waist deep in an ice-cold lake and shouting to the shore, "Come on in! The water's fine!" The fun of it, in both cases, was watching your friends' expressions when they discovered you were only luring them into making the same mistake you had done. He had to fight back a laugh as John did the equivalent of leaping in with both feet. Lending a phone to Sherlock! Might as well hand him your personal diary and then dance naked whilst he perused it.
That alone would have been enough to make the afternoon amusing. But he had to fight the laughter all the harder when Sherlock rattled off a string of deductions about John's 'brother'. Harry, a brother? Seeing Sherlock Holmes play the fool was icing on a most satisfactory cake. But with a potential flatshare hanging in the balance it wouldn't do to give the man any indication that he was wrong.
His only regret, Stamford thought, as he strolled away from the lab, was that he wouldn't be able to see Sherlock's face when he found out!